Fix StatsD server port for test server in spec (#1073)

Set the port as an instance on the server instance, and not rely on the
env var to be read in the thread spawned during the test. It might not
be read properly.

[skip changeset]

spec/lib/puma/appsignal_spec.rb

@@ -32,10 +32,14 @@ def error(message)
   # StatsD server used for these tests.
   # Open a UDPSocket and listen for messages sent by the AppSignal Puma plugin.
   class StatsdServer
+    def initialize(statsd_port)
+      @statsd_port = statsd_port
+    end
+
     def start
       stop
       @socket = UDPSocket.new
-      @socket.bind("127.0.0.1", ENV.fetch("APPSIGNAL_STATSD_PORT"))
+      @socket.bind("127.0.0.1", @statsd_port)
 
       loop do
         # Listen for messages and track them on the messages Array.
@@ -116,7 +120,7 @@ def in_background(&block)
   def run_plugin(stats_data, plugin, &block)
     Puma._set_stats = stats_data
     ENV["APPSIGNAL_STATSD_PORT"] = "8126"
-    @statsd = StatsdServer.new
+    @statsd = StatsdServer.new(ENV.fetch("APPSIGNAL_STATSD_PORT"))
     @server_thread = Thread.new { @statsd.start }
     @server_thread.abort_on_exception = true
     @client_thread = Thread.new { start_plugin(plugin) }